I said in this thread that my iPad Pro 9.7 deactivated out of the blue on iOS 9:
https://forums.macrumors.com/threads/ios-9-activation-error-ipad-pro-9-7.2198681/#post-27789526.
I tried a lot of things. I tried iMazing’s exit setup assistant, multiple hard resets, a tool from the jailbreak community, nothing worked. My iPad wouldn’t activate. In the end, Apple’s activation servers are broken on 64-bit devices on iOS 9, and I found a LOT of people that said they had to update. After hours of trying, I gave up. I updated to iOS 12.4.1. Performance is fine, other than a little keyboard lag in notes (weirdly, I am typing as fast as I can, and it doesn’t lag here or elsewhere). Also, performance on everything else is like-new.
Now, battery life took a MASSIVE dive. I’m a light user. My iPad got 13-14 hours when new on iOS 9. It was getting 12-13 before updating, with 84% health.
I use the iPad lightly, and it isn’t lasting 10 hours. Not even ten. Don’t say that it’s battery health, clearly it was fine on iOS 9.
Also: I don’t even want to imagine what it does to older iPhones... I guess what I thought and witnessed on a friend’s iPhone 7 was true. It barely scraped four hours, down from 8 initially on iOS 10.